Use of hydrocarbons as fuel for vehicles or plant operations (e.g. for heating systems, furnaces, burner and the like) involves extraction, refining, transport and storage of gas, diesel and other hydrocarbons typically known as being volatile. Because numerous steps are involved from the extraction to the final use of these volatile hydrocarbons, incidental or unintentional spillage often occurs, thereby leading to contamination of soils surrounding the sites where these fuel production steps are carried out. As existing environment legislation generally prohibit disposal of hazardous materials, soils and sludges contaminated by hydrocarbon fuels have to undergo decontamination steps to remove hydrocarbons for compliance with pollution restrictions and disposal.
Technologies for decontaminating soils and sludges are known in the art and can be broadly regrouped into three (3) categories namely bio-treatment, wash out and thermal treatment. Bio-treatment of contaminated soils typically consist in adding cellulose material such as wood chips or straw to adjust the moisture content of the substrate or soil, as well as feeding the same in nutriments and oxygen to encourage bacterial activity to thereby eliminate contaminants. Together with the fact that bio-treated soils tend not to be fully decontaminated, the use of cellulose additives contributes to make the decontaminated soils difficult to recycle as residential or commercial construction materials. As a consequence, bio-treated soils are generally disposed of in landfills. Further, bio-treatment of contaminated soils tends to be a very long process, requiring generally between three (3) and nine (9) months to reach an environmentally acceptable hydrocarbon content, and is associated with microbial production of significant amounts of the greenhouse gas carbon dioxide (CO2).
Wash out processes generally involve using a chemical surfactant solution for leaching or washing the contaminants into the aqueous matrix of the soil, collecting the contaminated aqueous matrix and treating the same. The contaminated liquid fraction is collected and itself submitted to treatment or decontamination steps. In addition to requiring substantial amount of expensive chemicals, wash out processes require specialized equipment such as sealed thanks water treatment systems. Wash out processes also tend to be of limited efficacy, see useless, where the soil to be treated includes fine particles (e.g. with clays and silteous soils). Further, soils decontaminated according to these processes tend to become water-saturated and are therefore of a limited use as construction material, especially when fine particles are present, such fine particles generally requiring further decontamination steps. Finally, because it is generally not possible to prevent evaporation of the solvents use for leaching the contaminants from the soil, recuperation and incineration thereof is often necessary, which also results in production of significant amounts of CO2.
Thermal treatments of contaminated soils may alleviate some drawbacks associated with bio-treatments and wash out processes since they tend to be suitable for most types of soils and generally permit complete decontamination of the treated soils. Thermal treatments include pyrolysis, incineration and thermal desorption.
Incineration requires heating large amounts of soils to very high temperatures, i.e. from 400° C. to 1,000° C., to decompose even small amounts of contaminants. As such, incineration tends to be energy inefficient and consequently uneconomical. Pyrolysis is also a process by which contaminated soils are also exposed to very high temperature, with the exception that, the process is carried out in absence of oxygen. Similarly to incineration, pyrolysis involves high energy consumption and tends to be costly. Both technologies are however associated with production of CO2, and do not allow recycling of the treated soils as construction materials, causing them to be less interesting on an environmental standpoint.
Thermal desorption involves heating the contaminated soil under oxygen concentration and residence time to enable volatilization and separation of the contaminants from the soil while avoiding their thermal degradation. A number of in situ decontamination technologies have been reported in the art. In situ desorption technologies typically involve drilling a plurality of wells or holes on the contaminated site, inducing a flow of heated air in the drilled wells to force volatilization of the contaminants and collecting the volatilized contaminants at the surface of the soil for further treatment thereof. The in situ desorption technologies known in the art tend to be expensive, most of the time ineffective and to require substantial amount of time (i.e. from 6 to 18 months) to reach acceptable decontamination level. Further, due to the difficulty to efficiently collect the volatilized contaminants, in situ technologies may result in contaminant escaping the collection systems and to remain in the atmosphere.
To alleviate the drawbacks associated to in situ technologies, some have proposed thermal desorption technologies carried off the contamination site, also know as ex situ thermal desorption. Ex situ desorption technologies typically involve excavation and transport of the contaminated soil to a treatment facility. Once at the treatment facility, the contaminated soil is placed in a treatment stockpile where a flow of heated air is circulated through the contaminated soil to volatilize the contaminant. The volatilize contaminant is collected and further treated.
Examples of such ex situ technologies are described in U.S. Pat. No. 5,067,852 to Plunkett (the '852 Patent), U.S. Pat. No. 5,836,718 to Price (the '718 Patent) and U.S. Pat No. 5,213,445 to Ikenberry (the '445 Patent). The '852 and '445 Patents both disclose methods and systems for removing contaminants from a soil by volatilization. The systems taught in these patents make use of pipe arrays embedded in a stockpile, which stockpile is further covered with flexible air-impermeable membranes or liners. The '852 and '445 Patents both make use of vacuum to encourage volatilization whereas the '718 patent uses hot pipes. According to some, these technologies would have been proven to be ineffective for effecting decontamination because of the low temperatures used to carry out the process without damaging the sealing members. Because the pipe arrays and/or membranes have to be dissembled or removed between batches of soil to be treated, those systems are not convenient.
Other example of ex situ technologies are described in U.S. Pat. No. 5,253,597 to Swanstrom et al. (the '597 Patent) and U.S. Pat No. 6,000,882 to Bova and Richter (the '882 Patent). These two patents describe methods and systems for causing volatilization of contaminants from a soil, where very high temperature and/or negative pressure (i.e. vacuum) are required to carry out the methods. As such, equipment must be adapted to sustain rigorous operating conditions and the systems tend not to be economical. An additional example of ex situ technologies is described in U.S. Pat. No. 6,881,009 to Stegemeir, this system using electrical resistance heater elements to heat the soil to be treated.
Further, the technologies described in the '852, '718, '445 '597 and '882 Patents, tend to be impractical or non convenient for treating large amounts of contaminated soils, either because de configuration of the vessels receiving the soil to be treated does not enable receiving large volume of soil or because re-used of the system require disassembly of the pipe arrays.
It would therefore be advantageous to be provided with an improved energy efficient system for treating contaminated soils ex situ. More preferably, such a system would allow sequential treatment of large volumes of contaminated soils without the need to dismantle substantial components thereof.